Method and apparatus for partitioning a content addressable memory device

ABSTRACT

A CAM device having a plurality of CAM blocks is partitioned into a number of individually searchable partitions, where each partition may include one or more CAM blocks of the CAM device. In one embodiment, each CAM block is connected to a block select circuit that stores a class code indicating what class or type of data is stored in the block. The same class code may be stored in any number of block select circuits to define a partition as including the corresponding number of CAM blocks. During compare operations between a comparand word and data stored in the CAM device, a search code is provided to the block select circuits. Each block select circuit compares the search code with its class code and, in response thereto, selectively enables or disables the corresponding CAM block for the compare operation. In some embodiments, the block select circuit enables the corresponding CAM block if the search class matches the class code and, conversely, disables the corresponding CAM block if the search code does not match the class code.

BACKGROUND

1. Field of Invention

This invention relates generally to semiconductor memories andspecifically to content addressable memories.

2. Description of Related Art

Content addressable memories (CAMs) are frequently used for addresslook-up functions in Internet data routing. For example, routers used bylocal Internet Service Providers (ISPs) typically include one or moreCAMs for storing a plurality of Internet addresses and associated datasuch as, for instance, corresponding address routing information. Whendata is routed to a destination address, the destination address iscompared with all CAM words, e.g., Internet addresses, stored in the CAMarray. If there is a match, routing information corresponding to thematching CAM word is output and thereafter used to route the data.

A CAM device includes a CAM array having a plurality of memory cellsarranged in an array of rows and columns. Each memory cell stores asingle bit of digital information, i.e., either logic zero or logic one.The bits stored within a row of memory cells constitute a CAM word.During compare operations, a comparand word is received at appropriateinput terminals of a CAM device and driven into the CAM array usingcomparand lines to be compared with all the CAM words in the device. Foreach CAM word that matches the comparand word, a corresponding matchline signal is asserted to indicate a match condition. If the comparandword matches more than one of the CAM words, the match linecorresponding to each of the matching CAM words is asserted, and a“multiple match” flag is also asserted to indicate the multiple matchcondition. The match line signals from each CAM block are combined in apriority encoder to determine the index or address of thehighest-priority matching CAM word. Associative informationcorresponding to the highest-priority matching CAM word stored in, forinstance, an associated RAM, may also be provided.

A single CAM device can be used to store multiple tables each storingand maintaining different classes of data. All entries, however,typically participate in a compare operation. This can cause anundesirable amount of power to be drawn during the compare operation. Itwould be desirable to limit a search to only those entries associatedwith a particular class of data to reduce power consumption during theoperation.

SUMMARY

A method and apparatus are disclosed that may be used to partition a CAMdevice having a plurality of CAM blocks into a number of individuallysearchable partitions, where each partition may include one or more CAMblocks of the CAM device. In accordance with one embodiment of thepresent invention, each CAM block is connected to a block select circuitthat stores a class code indicating what class or type of data is storedin the block. The same class code may be stored in any number of theblock select circuits to define a partition as including thecorresponding number of CAM blocks. During compare operations between acomparand word and data stored in the CAM device, a search code isprovided to the block select circuits. Each block select circuitcompares the search code with its class code and, in response thereto,selectively enables or disables the corresponding CAM block for thecompare operation. In some embodiments, the block select circuit enablesthe corresponding CAM block if the search class matches the class codeand, conversely, disables the corresponding CAM block if the search codedoes not match the class code.

In one embodiment, the block select circuit disables a corresponding CAMblock by driving the comparand lines of the CAM block to a predeterminedstate to preclude the comparand word from being driven onto thecomparand lines during the compare operation. By driving the comparandword only on the comparand lines of the selected (i.e., enabled) CAMblocks during the compare operation, present embodiments not only allowfor selective searching across CAM blocks according to class codes, butalso reduce power consumption in un-selected (i.e., disabled) CAM blocksduring such selective compare operations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a CAM device including a plurality of CAMblocks in accordance with one embodiment of the present invention;

FIG. 2 is a block diagram of a CAM block in one embodiment of the arrayof FIG. 1;

FIG. 3 is a block diagram of a block select circuit in one embodiment ofthe CAM block of FIG. 2;

FIG. 4 is a logic diagram of one embodiment of the block select circuitof FIG. 3;

FIG. 5 is a logic diagram of another embodiment of the block selectcircuit of FIG. 3;

FIG. 6 is a logic diagram of one embodiment of a comparand driver of theCAM block of FIG. 2;

FIG. 7 is a block diagram of a CAM device including a plurality of CAMblocks in accordance with another embodiment of the present inventionconfigured to translate addresses of one or more defective CAM blocks;

FIG. 8 is a flow chart illustrating the disabling of defective CAMblocks in one embodiment;

FIG. 9 is a block diagram of one embodiment of the CAM device of FIG. 7;

FIG. 10 is a block diagram of a CAM block in one embodiment of thedevice of FIG. 9;

FIG. 11 is a block diagram of address translation logic in oneembodiment of the CAM block of FIG. 10;

FIG. 12 is a logic diagram of a main priority encoder in one embodimentof the device of FIG. 9.

Like reference numerals refer to corresponding parts throughout thedrawing figures.

DETAILED DESCRIPTION

Embodiments of the present invention are discussed below in the contextof a CAM device 100 for simplicity only. It is to be understood thatembodiments of the present invention are equally applicable to CAMstructures having other configurations of any suitable type of CAMcells. Further, architectural configurations of the present inventionmay be implemented in other types of memory blocks such as, forinstance, RAM, Flash, and EEPROM. The interconnection between circuitelements or blocks may be shown as buses or as single signal lines,where each of the buses may alternatively be a single signal line, andeach of the single signal lines may alternatively be a bus. In addition,the logic levels assigned to various signals in the description beloware arbitrary, and therefore may be modified (e.g., reversed polarity)as desired. Accordingly, the present invention is not to be construed aslimited to specific examples described herein but rather includes withinits scope all embodiments defined by the appended claims.

FIG. 1 shows a CAM device 100 in accordance with one embodiment of thepresent invention as having a number n of CAM blocks 102(1)-102(n), acorresponding number n of block select circuits 106(1)-106(n), and apriority encoder 108. Each CAM block 102 includes a CAM array 104 havinga plurality of rows of CAM cells for storing a plurality of CAM wordstherein, and is connected to a corresponding block select circuit 106.Each row may also include one or more valid bits indicative of whether avalid CAM word is stored in the row. The valid bits may be used in awell-known manner to generate a full flag for the CAM block 102. CAMblocks 102 may be any suitable type of CAM block, including for example,synchronous, asynchronous, binary, and ternary CAMs. Further, each CAMblock 102 may be any suitable size, and in some embodiments may be ofdifferent sizes. In one embodiment, each CAM block 102 includes 1k(1024) rows of CAM cells.

During a compare operation, each CAM block 102 receives comparand datafrom a comparand bus CBUS. Other signals provided to the CAM device 100during the compare operation may be a clock signal CLK, one or moreinstructions from an instruction decoder (not shown for simplicity), andother control signals. In some embodiments, instructions and comparanddata may be provided to the CAM blocks 102(1)-102(n) via the same bus.Other well-known signals which may be provided to the CAM blocks 102,such as word enable signals, reset signals, and enable signals, are notshown for simplicity.

Each CAM block 102 provides a plurality of match line signals to thepriority encoder 108 via corresponding match lines ML. The match linescarry match signals indicative of match conditions in the CAM arrays104. For simplicity, the plurality of match lines ML from each CAM block102 are represented collectively in FIG. 1. The priority encoder 108generates an index corresponding to one of the matching CAM words in thedevice 100. In one embodiment, the priority encoder 108 outputs theindex of the highest priority match. The highest priority match may bethe lowest numbered address, the highest numbered address, or any otherselected address.

For purposes of discussion herein, the first CAM block 102(1) in thedevice 100 is designated as the highest priority block, the second CAMblock 102(2) is designated as the next highest priority block, and soon, and the last CAM block 102(n) is designated as the lowest priorityblock, although in actual embodiments priority may be reversed orotherwise modified. Thus, the highest priority CAM block 102(1) mayinclude the lowest CAM addresses (i.e., CAM addresses 0 to k−1), thenext highest priority block 102(2) may include the next lowest CAMaddresses (i.e., CAM addresses k to 2k−1), and so on, and the lowestpriority CAM block 102(n) may include the highest CAM addresses (i.e.,CAM addresses (n−1)k to nk−1.

The block select circuits 106(1)-106(n) control whether correspondingCAM blocks 102(1)-102(n), respectively, participate in compareoperations. Each block select circuit 106 stores a class code for thecorresponding CAM block 102 which may be used to selectively disable theCAM block from participating in, and therefore from affecting theresults of, one or more compare operations. During a compare operation,a comparand word is provided to the CAM blocks 102 via CBUS, and asearch code is provided to the block select circuits 106(1)-106(n) viabus SC. In alternate embodiments, the search code may be provided aspart of the comparand word, in which case the CBUS is connected to theblock select circuits 106, or may be provided as part of a compareinstruction. Each block select circuit 106 compares the received searchcode with its stored class code, and in response thereto, selectivelydisables the corresponding CAM block 102 from participating in thecompare operation via a select signal SEL. In one embodiment, the blockselect circuit 106 enables its corresponding CAM block 102 toparticipate in the compare operation if the class code matches thesearch code and, conversely, disables the corresponding CAM block 102 ifthe class code does not match the search code. In alternate embodiments,more than one CAM block 102 may share the same block select circuit 106.

The class codes assigned to the CAM blocks 102 may be used to partitionthe device 100 into individually selectable partitions of one or moreCAM blocks 102. For example, in one embodiment, data stored in the firstCAM block 102(1) may be assigned to a first class by storing a firstclass code in block select circuit 106(1), data stored in the second CAMblock 102(2) may be assigned to a second class by storing a second classcode in block select circuit 106(2), and data stored in the remainingCAM blocks 102(3)-102(n) may be assigned to a third class by storing athird class code in block select circuits 106(3)-106(n). Then, forexample, data stored in the first CAM block 102(1) may be selected forsearching by setting the search code to match the first class codestored in the block select circuit 106(1).

When the search code matches the first class code, the block selectcircuit 106(1) enables the first CAM block 102(1) to compare thecomparand word with its stored data corresponding to the first classcode. If the search code does not match the second and third classcodes, the remaining block select circuits 106(2)-106(n) disable thecorresponding, unselected CAM blocks 102(2)-102(n). When disabled, theunselected CAM blocks 102(2)-102(n) do not drive the comparand word intotheir respective CAM arrays 104 for the compare operation, therebyprecluding comparison with unselected data corresponding to the secondand third class codes. In this manner, the CAM blocks 102(1)-102(n) maybe selectively searched according to class assignments, thereby allowingfor a dynamically partition-able CAM device 100.

Since the comparand word is not compared with data stored in thedisabled CAM blocks 102(2)-102(n), the disabled CAM blocks 102(2)-102(n)consume much less power during the compare operation than does theselected, enabled CAM block 102(1). In this manner, the class codes ofpresent embodiments not only restrict compare operations to data in theselected CAM block(s), but also minimize power consumption of theunselected CAM block(s) during compare operations. The advantage ofreduced power consumption in unselected CAM blocks during compareoperations achieved by present embodiments may be particularly useful inapplications where power consumption is a concern.

The ability to selectively enable or disable one or more CAM blocks fromparticipating in compare operations may be especially useful forcombining routing look-up functions for different classes of networks ina single device 100. For example, in one embodiment, routing informationfor a first virtual private network (VPN) may be stored in a first CAMblock 102(1), routing information for a second VPN may be stored in asecond CAM block 102(2), routing information for a web search may bestored in a third block 102(3), and routing information for a local areanetwork (LAN) may be stored in a fourth CAM block 102(4). Four uniqueclass codes may be stored in corresponding block select circuits 106. Ofcourse, more than one CAM block may be assigned to a particular networkby storing the appropriate class code in more than block select circuit106. During compare operations, comparand data corresponding to routingfunctions of one of these four networks may be exclusively compared withdata stored in the corresponding CAM block(s) by simply setting thesearch code to match the appropriate class code. In some embodiments, anassociative RAM may be partitioned into four partitions correspondingwith the four class-defined partitions in the CAM device 100.

FIG. 2 shows a CAM array 200 that is one embodiment of a CAM array 104of FIG. 1. The array 200 includes a plurality of CAM cells 202 organizedin any number of rows and columns. Each row of CAM cells 202 is coupledto a match line ML and a word line WL. Each word line WL is driven by anaddress decoder 204 to select one or more of CAM cells 202 for writingor reading. For alternative embodiments, multiple CAM blocks may share adecoder. Each match line ML provides the match results of a compareoperation to the priority encoder 108 (see also FIG. 1). A match line MLindicates a match condition for the row only if all CAM cells 202 inthat row match the comparand data. Each CAM cell 202 may be a binary,ternary, SRAM-based or DRAM-based CAM cell. In some embodiments, thematch line ML is pre-charged for the compare operation. If any CAM cell202 in the row does not match the comparand data, the CAM cell(s) 202discharges the match line ML toward ground potential (e.g., logic low).Conversely, if all CAM cells 202 match the comparand data, the matchline ML remains in a charged state (e.g., logic high). When the CAMblock 102 is disabled in response to the select signal SEL, thecomparand word is not driven into the array 200, and the match lines MLmay remain in their charged state during the compare operation,regardless if there is a mismatch. The match lines need not bepre-charged for a subsequent compare operation. The ability to maintainthe match lines of unselected CAM blocks in their charged state duringthe compare operation may further reduce power consumption of presentembodiments over prior art architectures.

Each column of CAM cells 202 is coupled to a bit line BL, acomplementary bit line {overscore (BL)}, a comparand line CL, and acomplementary comparand line {overscore (CL)}. The bit lines BL and{overscore (BL)} are coupled to sense amplifiers 206 that may enabledata to be written to or read from a row of CAM cells 202. The comparandlines CL and {overscore (CL)} are coupled to comparand drivers 208,which in turn are coupled to a comparand register 210 via complementarydata lines D and {overscore (D)}. The comparand drivers 208 selectivelydrive a comparand word received from the comparand register 210 viacomplementary data lines D and {overscore (D)} onto complementarycomparand lines CL and {overscore (CL)} for comparison with data in CAMcells 202 in response the select signal SEL provided by the block selectcircuit 106. The comparand register 210 may be shared by all CAM blocks102(1)-102(n). As discussed above with respect to FIG. 1, the blockselect circuit 106 generates the select signal SEL in response to thesearch code and its stored class code.

In alternate embodiments, other CAM array architectures may be used. Forexample, in some embodiments, CAM array 200 may not includecomplementary comparand lines CL and {overscore (CL)}, in which case thecomplementary bit lines BL and {overscore (BL)} may be coupled to thecomparand drivers 208 and be used to perform a compare operation as isgenerally known in the art. For example, in the first part of a comparecycle, compare data may be selectively driven onto BL and {overscore(BL)}, and during the second part of the compare cycle, BL and{overscore (BL)} may be driven with data to be output from CAM array200. For other embodiments, only one of comparand lines CL and{overscore (CL)} or bit lines BL and {overscore (BL)} may be needed.

FIG. 3 shows a block select circuit 300 that is one embodiment of theblock select circuit 106. The block select circuit 300 includes a memory302 and a compare circuit 304. The memory 302 stores the class code forthe corresponding CAM block 102 of device 100, and may be any suitableprogrammable memory element such as, for instance, a register,flip-flop, EEPROM, EPROM, SRAM, and so on. The compare circuit 304compares the class code received from the memory 302 with a search codereceived from bus SC and, in response thereto, generates the selectsignal SEL which selectively enables or disables the corresponding CAMblock 102. The compare circuit 304 may be any suitable circuit whichcompares the search code and the class code, including for example anexclusive-OR type logic gate or a CAM cell.

FIG. 4 shows a block select circuit 400 that is one embodiment of theblock select circuit 300. The block select circuit 400 is shown toinclude a 3-bit memory 302 and a 3-bit compare circuit 304, although inother embodiments more or less bits may be used. The memory 302 includesthree data flip-flops 402(0)-402(2), and the compare circuit 304includes three exclusive-NOR (XNOR) gates 404(0)-404(2) and an AND gate406. Each XNOR gate 404(0)-404(2) includes a first input terminal toreceive a corresponding search code bit SC, a second input terminal toreceive a corresponding class code bit CC from the correspondingflip-flop 402, and an output terminal connected to the AND gate 406. A3-bit class code CC[0:2] may be clocked into respective flip-flops402(0)-402(2) using the clock signal CLK, where flip-flop 402(0) storesthe first class code bit CC[0], flip-flop 402(1) stores the second classbit code CC[1], and flip-flop 402(2) stores the third class code bitCC[2].

During compare operations, the XNOR gates 404(0)-404(2) compare searchcode bits SC[0:2] with respective class code bits CC[0:2] and, if thereis match, drive their output terminals to logic high. Conversely, ifthere is a mismatch, the XNOR gate 404 drives its output terminal tologic low. If all search code bits SC[0:2] match corresponding classcode bits CC[0:2], then AND gate 406 asserts the select signal SEL tologic high, thereby enabling the corresponding CAM block 102 toparticipate in the compare operation. Otherwise, if any of the searchcode bits SC[0:2] mismatch corresponding class code bits CC[0:2], theAND gate 406 de-asserts the select signal to logic low, therebydisabling the corresponding CAM block 102 from participating in thecompare operation. Since class code bits may be loaded into flip-flops402(0)-402(2) before a compare operation, the gate delay associated withgenerating the select signal during the compare operation is only 2 gatedelays, one for XNOR gates 404 and one for AND gate 406, and thereforehas a negligible effect upon device performance.

FIG. 5 shows a block select circuit 500 that is another embodiment ofthe block select circuit 300. Here, a logic circuit 502 is coupled tothe output terminal of the AND gate 406 to allow for direct control ofthe select signal SEL using control signals EN and SEL_OV. The logiccircuit 502 includes an OR gate 504 having a first terminal coupled tothe output terminal of the AND gate 406, a second terminal to receiveEN, and an output terminal coupled to a first input terminal of an ANDgate 508. The AND gate 508 includes a second input terminal to receiveSEL_OV, and an output terminal to provide the select signal SEL. Thesignal EN enables the corresponding CAM block 102 to participate in thecompare operation when the output of AND gate 406 is logic high. Whenasserted to logic high, EN enables the corresponding CAM block 102 forthe compare operation and, conversely, when de-asserted to logic low, ENdisables the corresponding CAM block 102 for the compare operationregardless if there is a match condition. The EN signal may be used toselectively disable CAM blocks 102, for instance, when defective. Thesignal EN is shown in FIG. 5 as being provided by a fuse 506, althoughin other embodiments EN may be provided by other means such as aprogrammable memory element, e.g., a register, flip-flop, EPROM, EEPROM,SRAM, etc. The signal SEL_OV is a select override signal that, whenasserted to logic high, may be used to force the select signal SEL tologic high to enable the corresponding CAM block 102 to participate incompare operations, irrespective of whether there is a class match. Foran alternative embodiment, the relative locations of OR gate 504 and ANDgate 508 may be reversed such that when SEL_OV is set to a logic highstate, then SEL will be set to a logic high state irrespective ofwhether there is a class match or the logic state of EN.

FIG. 6 shows a 1-bit comparand driver 600 that is used in one embodimentof the comparand drivers 208. Driver 600 includes AND gates 602, 604,and 606, and also includes buffers 608 and 610. AND gate 602 includesinput terminals to receive the clock signal CLK and the select signalSEL, and an output terminal coupled to first input terminals of ANDgates 604 and 606. AND gate 604 includes a second input terminal coupledto the data line D, and an output terminal coupled to the buffer 608,which in turn drives the comparand line CL. AND gate 606 includes asecond input terminal coupled to the complementary data line {overscore(D)}, and an output terminal coupled to the buffer 610, which in turndrives the complementary comparand line {overscore (CL)}. Buffers 608and 610 may be any suitable buffers to drive comparand data onto thecomparand lines CL and {overscore (CL)}. A plurality of drivers 600 mayshare the AND gate 602.

During a compare operation, a comparand bit is provided to AND gate 604via data line D, and a complementary comparand bit is provided to ANDgate 606 via complementary data line {overscore (D)}. When CLK is logichigh, the select signal SEL propagates through AND gate 602 to AND gates604 and 606. If the select signal is asserted to logic high, AND gate606 passes the comparand bit to the buffer 608, which in turn drives thecomparand bit onto the comparand line CL. Similarly, AND gate 608 passesthe complementary comparand bit to the buffer 610, which in turn drivesthe complementary comparand bit onto the complementary comparand line{overscore (CL)}. Thus, when the select signal SEL is asserted, thecomparand driver 600 drives the comparand lines CL and {overscore (CL)}with the comparand data received from the comparand register 210 viadata lines D and {overscore (D)}.

Conversely, if the select signal SEL is de-asserted to logic low toindicate that the corresponding CAM block 102 is not to participate inthe compare operation, AND gates 606 and 608 force their respectiveoutput terminals to logic low. In response thereto, buffers 608 and 610force the comparand line CL and the complementary comparand line{overscore (CL)}, respectively, to logic low. In this manner, when theselect signal SEL is de-asserted, the comparand driver 600 does notdrive complementary comparand data onto the comparand lines CL and{overscore (CL)}, thereby precluding the corresponding CAM block 102from participating in the compare operation while minimizing powerconsumption in the CAM block.

The present invention is also particularly useful in increasingmanufacturing yield of a CAM device by disabling defective CAM blocks inthe device. Thus, for instance, during manufacture of a CAM devicehaving n CAM blocks, if one or more of the CAM blocks are found to bedefective or otherwise inoperable after manufacturing, rather thandiscarding the entire device, the defective blocks may be disabled usingthe block select circuits as described above, and the remainingnon-defective CAM blocks may then be used for compare operations. Forexample, in one embodiment where the CAM device includes 8 CAM blockseach having 1k rows of CAM cells, if one of the CAM blocks is defective,that CAM block is disabled, and the remaining 7 CAM blocks may be usedas a 7k CAM device. Accordingly, the ability to use the CAM device whenone or more of its CAM blocks are defective advantageously increasesmanufacturing yield of the CAM device.

FIG. 7 shows a CAM device 700 that is a modified embodiment of thedevice 100 of FIG. 1 which allows for one or more defective CAM blocksto be disabled for CAM operations, and also includes circuitry whichtranslates or re-assigns address locations in defective CAM blocks toaddress locations in non-defective CAM blocks. The device 700 includesaddress logic 701, a plurality of CAM blocks 702(1)-702(n), a pluralityof block select circuits 706(1)-706(n) corresponding to CAM blocks702(1)-702(n), respectively, a priority encoder 708, and full flag logic710. Each of the block select circuits 706(1)-706(n) provides to thecorresponding CAM block 702 a select signal which may be used asdescribed above to disable the CAM block 702 if, for example, the CAMblock 702 is defective.

The block select circuit 706 may be any suitable circuit to provideeither a logic high (enabling) or a logic low (disabling) select signalSEL to the corresponding CAM block 702. In some embodiments, the blockselect circuit 706 includes a memory (not shown in FIG. 7) for storing abinary value indicative of SEL. In some embodiments, the block selectcircuit 706 provides a logic high SEL signal if the corresponding CAMblock 702 is not defective, and provides a logic low SEL signal if thecorresponding CAM block 702 is defective. In one embodiment, the blockselect circuit 706 may include the block select circuit 500 (FIG. 5), inwhich case the signal EN may be set to a low logic state by blowing fuse506 to disable a defective CAM block 702 via signal SEL. In otherembodiments, the block select circuit 706 may be a fuse (or a memoryelement) connected between the CAM block 702 and a voltage supply, inwhich case the fuse may be blown to provide a logic low SEL signal todisable the corresponding CAM block 702.

After fabricating the device 700, each of the CAM blocks 702(1)-702(n)is tested in a suitable manner. For each CAM block 702 that is found tobe defective, the corresponding block select circuit 706 is configuredto provide a logic low select signal SEL to the CAM block 702 to disablethe CAM block. Conversely, for each CAM block 702 that is not defective,the corresponding block select circuit 706 is configured to provide alogic high select signal to the CAM block to enable its participation inCAM operations.

Testing the CAM blocks of a CAM device and then selectively disablingthe defective CAM blocks in an embodiment using a fuse to provide SEL isillustrated with reference to the flow chart of FIG. 8. Here, a fuse(not shown for simplicity) in each block select circuit 706 is coupledto a voltage supply and thus initially provides an asserted (e.g., logichigh) SEL to enable the corresponding CAM block 702. Each CAM block 702is tested in a suitable manner to determine whether it is defective(step 750). If the CAM block is defective, as tested at step 751, SEL isde-asserted (e.g., to logic low) to disable the defective CAM block byblowing the fuse. Otherwise, if the CAM block is not defective, thecorresponding fuse is not blown, and the corresponding CAM block remainsenabled. If all CAM blocks have been tested, as determined at step 753,processing is finished (step 754). Otherwise, the next CAM block istested and thereafter disabled if found to be defective (steps 750-752).

During a compare operation, each CAM block 702 receives comparand datafrom the comparand bus CBUS in a manner similar to that of CAM blocks102 of device 100 of FIG. 1. Other signals provided to device 700 duringthe compare operation may be a clock signal CLK, one or moreinstructions from an instruction decoder (not shown for simplicity), andother control signals. Each CAM block 702 provides a plurality of matchline signals to the priority encoder 708 via corresponding match linesML. The match lines carry match signals indicative of match conditionsin the CAM arrays 704. For simplicity, the plurality of match lines MLfrom each CAM block 702 are represented collectively in FIG. 7. Thepriority encoder 708 generates an index corresponding to one of thematching CAM words in the device 700, which as described above may beindex of the highest-priority matching CAM row.

Each CAM block 702 provides a full flag signal FF indicative of whetherthe CAM block is full, i.e., whether there are any available row in theCAM block 702 to store data, to full flag logic 710. The full flagsignal FF may be generated for each CAM block 702 in a well-known mannerusing one or more valid bits in each row of the CAM block. The full flagsignals FF_1 to FF_n provided by CAM blocks 702(1)-702(n), respectively,are combined in a well-known manner in full flag logic 710 to generate adevice full flag, FF_device, indicative of whether there are anyavailable rows in the device 700. When a CAM block 702 is found to bedefective or otherwise inoperable for its intended purpose, the CAMblock 702 is configured to maintain an asserted full flag signal FF toindicate that the defective CAM block 702 does not include any availablememory locations. In one embodiment, the full flag signal FF for thedefective CAM block may be maintained in the asserted state by forcingthe valid bits in its array 704 to an asserted state. In otherembodiments, a fuse may be provided within or associated with each CAMblock 702 that, when blown, forces the corresponding full signal FF tobe asserted.

Address logic 701 is shown in FIG. 7 as coupled to an address bus ABUSand each of the CAM blocks 702(1)-702(n). During read and writeoperations, an address provided to the device 700 may be received intoaddress logic 701 via address bus ABUS, and thereafter used to select arow in one of the CAM blocks 702(1)-702(n) for the read or writeoperation. In accordance with the present invention, if a CAM block 702to which the address refers is defective, and is thus disabled for theoperation using the corresponding block select circuit 706 as describeabove, address logic 701 translates the address from the defective ordisabled CAM block to a non-defective CAM block. Conversely, if the CAMblock 702 to which the address refers is non-defective, and is thusenabled for operation, address logic 701 forwards the address to theappropriate CAM block 702. As explained more fully below, address logic701 ensures a contiguous addressing scheme in the CAM blocks 702 whenone or more CAM blocks 702 are defective and disabled, even when thenon-defective CAM block(s) 702 are not adjacent to each other.

For alternate embodiments, address logic 701 may be omitted. For oneexample, contiguous non-defective blocks starting from block 702(1) maystill be used. For other embodiments, any non-defective block may beused.

FIG. 9 shows a CAM device 800 that is one embodiment of the CAM device700. CAM device 800 is shown to include address translation logic 801,four CAM blocks 802(0)-802(3), and a main priority encoder 806. Each CAMblock 802 includes a CAM array 704 (e.g., a 1k CAM array), a blockpriority encoder 804, and match flag logic 805. Of course, in otherembodiments, there may be any number of CAM blocks 802, and each CAMblock array 704 may include any number of rows of CAM cells. The addressA may include any suitable number of bits. In the embodiment of FIG. 9,the function of the priority encoder 708 of FIG. 7 is distributedbetween the individual block priority encoders 804 within the CAM blocks802(0)-802(3) and the main priority encoder 806. During a read or writeoperation, a 14-bit address A[13:0] may be provided to the device 800via the address bus ABUS. The first two address bits A[13:12] are theblock address bits and are provided to address translation logic 801,which in turn selects one of the CAM blocks 802(0)-802(3) for the reador write operation via block select signals BS_0 to BS_3, respectively.The remaining twelve address bits, A[11:0], select a row in the CAMarray 704 selected by address translation logic 801 for the operation,and may be provided to each CAM block 802. During read or writeoperations, data may be read from or written to the row identified byrow address bits A[11:0] in the CAM block 802 selected by addresstranslation logic 801.

Information indicative of which CAM blocks 802 are found to be defectiveduring testing may be used to configure address translation logic 801 tore-address the non-defective CAM blocks 802 so as to occupy, forinstance, the contiguous highest-priority address space (e.g., thelowest numbered addresses). During a read or write operation, addresstranslation logic 801 receives block address bits A[13:12]. If a CAMblock 802 selected by block address bits A[13:12] is non-defective orotherwise enabled, address translation logic 801 asserts thecorresponding block select signal BS to enable the selected CAM block802 for the operation. For example, if an address [13:0] selects thefirst row in the first CAM block 802(0) for reading, and CAM block802(0) is non-defective, address translation logic 801 asserts BS_0 tologic high while maintaining BS_1, BS_2, and BS_3 in a logic low,de-asserted state. The asserted BS_0 signal causes row address bitsA[11:0] to be latched into the first CAM block 802(0), therebyfacilitating a read from the first CAM block 802(0).

Conversely, if a CAM block 802 selected by block address bits A[13:12]is defective or otherwise disabled, address translation logic 801selects another CAM block for the operation by asserting itscorresponding block select signal BS. For example, if the address [13:0]selects the first row in the first CAM block 802(0) for reading, and CAMblock 802(0) is defective and the second CAM block 802(1) isnon-defective, address translation logic 801 may assert BS_1 to logichigh while maintaining BS_0, BS_2, and BS_3 in a logic low, de-assertedstate. The asserted BS 1 signal causes row address bits A[11:0] to belatched into the second CAM block 802(1), thereby facilitating a readfrom the second CAM block 802(1). In this manner, address translationlogic 801 may re-address read or write operations from defective CAMblocks to non-defective CAM blocks.

In some embodiments, the block select signal BS provided to the CAMblock 802 may be used as an address gating signal to facilitate addresstranslation in accordance with present embodiments. For example, FIG. 10shows a CAM array 900 that is one embodiment of the array 704 of FIG. 7.The array 900 includes a plurality of CAM cells 202 organized in anynumber of rows and columns, and operates in a manner similar to the CAMarray 200 described above with respect to FIG. 2. That is, duringcompare operations, comparand data provided by the comparand register210 is selectively driven onto the complementary comparand lines CL and{overscore (CL)} in response to the select signal SEL provided by theblock select circuit 706. If the array 900 is non-defective, the selectsignal SEL is asserted to logic high to allow the comparand word to bedriven into the array 900 for comparison with CAM words stored therein.Conversely, if the array 900 is found to be defective during testing,the block select circuit 706 is configured to provide a de-assertedselect signal SEL to the comparand drivers 208 to prevent comparand datafrom being driven onto the comparand lines CL and {overscore (CL)},thereby disabling the array 900.

Address bits A[11:0] are provided from address bus ABUS to the addressdecoder 204. Address gating logic 902 is connected between the addressdecoder 204 and corresponding word lines WL of the array 900 via gatedlines GL, and selectively drives a word line WL identified by A[11:0] inresponse to the block select signal BS. For example, during a read orwrite operation, address decoder 204 decodes A[11:0] to select a row ofCAM cells 202 for the operation, and drives a corresponding gated lineGL to logic high. If BS is asserted to logic high, address gating logic902 drives the corresponding word line WL to select the row of CAM cells202 for the operation. Conversely, if BS is de-asserted to logic low,address logic 902 does not drive any of the word lines WL to logic high,regardless of A[11:0], thereby preventing CAM cells 202 in the arrayfrom being addressed for the operation.

In one embodiment, address gating logic 902 may include for each wordline WL in the array 900 an AND gate (not shown) having an outputterminal coupled to the word line, a first input terminal coupled to thecorresponding gated line GL, and a second input terminal to receive theblock select signal BS. In this manner, the AND gates may be used toselectively gate the addressing of CAM cells in the block in response toBS. Of course, in other embodiments other suitable logic may be used.

FIG. 11 shows address translation logic 1000 that is one embodiment ofthe address translation logic 801 of FIG. 9. Logic 1000 includes decodelogic 1002, four 4-input multiplexers 1004(0)-1004(3), and fourcorresponding memory elements 1006(0)-1006(3), respectively. Decodelogic 1002 has an input terminal to receive block address bits A[13:12],and has four output terminals coupled to corresponding input terminalsof the multiplexers 1004(0)-1004(3) via lines 1008(0)-1008(3),respectively. Decode logic 1002 decodes block address bits A[13:12], andin response thereto, asserts one of output lines 1008(0)-1008(3) tologic high. For example, if address bits A[13:12] are “00”, which isequivalent to the decimal value “0”, decode logic asserts line 1008(0);if address bits A[13:12] are “01”, which is equivalent to the decimalvalue “1”, decode logic 1002 asserts line 1008(1); if address bitsA[13:12] are “10”, which is equivalent to the decimal value “2”, decodelogic 1002 asserts line 1008(2); and if address bits A[13:12] are “11”,which is equivalent to the decimal value “3”, decode logic 1002 assertsline 1008(3).

The multiplexers 1004(0)-1004(3) each include an output terminal coupledto a corresponding one of the CAM blocks 802(0)-802(3), respectively,and a control terminal coupled to a corresponding one of the memoryelements 1006(0)-1006(3), respectively. Each memory element 1006 storesaddress translation information that when provided to the correspondingmultiplexer 1004 selects one of the signals provided by decode logic1002 to be output as the block select signal BS. In this manner,multiplexers 1004(0)-1004(3) may dynamically assign block address valuesto CAM blocks 802(0)-802(3), respectively.

In accordance with present embodiments, a read or write operation to adefective CAM block may be re-addressed to a non-defective CAM block bymanipulating the address translation information stored in the memoryelements 1006(0)-1006(3). In some embodiments, where all CAM blocks802(0)-802(3) are non-defective, the memory elements 1006(0)-1006(3)store default block address values so as to not alter CAM addressingduring the read or write operation. That is, the default block addressvalues cause respective multiplexers 1004(0)-1004(3) to selectcorresponding signals on lines 1008(0)-1008(3) as block select signalsBS_0 to BS_3, respectively. For example, memory element 1006(0) maystore a default block address value of “0” to cause multiplexer 1004(0)to select the signal on line 1008(0) as BS_0, memory element 1006(1) maystore a default block address value of “1” to cause multiplexer 1004(1)to select the signal on line 1008(1) as BS_1, memory element 1006(2) maystore a default block address value of “2” to cause multiplexer 1004(2)to select the signal on line 1008(2) as BS_2, and memory element 1006(3)may store a default block address value of “3” to cause multiplexer1004(3) to select the signal on line 1008(3) as BS_3. In this manner,address translation logic 1000 selects for the read or write operationthe CAM block identified by address bits A[13:12]. Table 1 summarizesthe four default block address/multiplexer select values (MUX) andcorresponding address space when all CAM blocks are non-defective.

TABLE 1 Block Status MUX address space CAM 802(0) non-defective 0 0 tok-1 CAM 802(1) non-defective 1 k to 2k-1 CAM 802(2) non-defective 2 2kto 3k-1 CAM 802(3) non-defective 3 3k to 4k-1

The CAM device 800 is then tested to determine if any CAM blocks aredefective. Where it is determined that one or more CAM blocks aredefective, the select values stored in memory elements 1006(0)-1006(3)may be modified to re-address the non-defective CAM blocks. For example,if after testing it is determined that CAM block 802(0) is defective,and is thereafter disabled using the corresponding block select circuit706 as described above (see also FIG. 7), the 1k CAM rows in thedefective CAM block 802(0) are no longer available, and therefore thedevice 800 now has only 3k available CAM rows available, i.e., 1k rowsin each of the 3 non-defective CAM blocks 802(1)-802(3). Since the firstCAM block 802(0) is not available, it is desirable for the second CAMblock 802(1) to be the highest-priority CAM block (e.g., having addressspace 0 to k−1), for the third CAM block 802(2) to be the secondhighest-priority CAM block (e.g., having address space k to 2k−1), andfor the fourth CAM block 802(3) to be the third highest-priority CAMblock (e.g., having address space 2k to 3k−1.

The block address values stored in corresponding memory elements1006(0)-1006(3) may be modified to implement a new addressing scheme forthe non-defective CAM blocks 802(1)-802(3). For example, in oneembodiment, the block address value stored in the memory element 1006(1)is set to “0” so that multiplexer 1004(1) selects the signal on line1008(0) as BS_1 to be provided to CAM block 802(1). When A[13:12] equals“00”, decode logic 1002 asserts line 1008(0) to logic high, which inturn now passes through multiplexer 1006(1) to select the second CAMblock 802(1) for the operation. In this manner, address translationlogic 1000 translates address space 0 to k−1 from CAM block 802(0) toCAM block 802(1).

Similarly, the block address value stored in the memory element 1006(2)is set to “1” so that multiplexer 1004(2) selects the signal on line1008(1) as BS_2 to provide to CAM block 802(2). When A[13:12] equals“01”, decode logic 1002 asserts line 1008(1) to logic high, which inturn now passes through multiplexer 1004(2) to select the third CAMblock 802(2) for the operation, thereby translating address space k to2k−1 from CAM block 802(1) to CAM block 802(2). Similarly, the blockaddress value stored in the memory element 1006(3) is set to “2” so thatmultiplexer 1004(3) selects the signal on line 1008(2) as BS_3 toprovide to CAM block 802(3). When A[13:12] equals “10”, decode logic1002 asserts line 1008(2) to logic high, which in turn now passesthrough multiplexer 1004(3) to select the fourth CAM block 802(3) forthe operation, thereby translating address space 2k to 3k−1 from CAMblock 802(2) to CAM block 802(3). Table 2 summarizes the select valueswhen CAM block 802(0) is defective and CAM blocks 802(1)-802(3) arenon-defective.

TABLE 2 Block Status MUX address space CAM 802(0) defective 3 3k to4k-1* CAM 802(1) non-defective 0 0 to k-1 CAM 802(2) non-defective 1 kto 2k-1 CAM 802(3) non-defective 2 2k to 3k-1 *not used

By translating address space in CAM blocks 802(1)-802(3), respectively,present embodiments may re-address rows in non-defective CAM blocks802(1)-802(3) with the highest-priority CAM addresses, e.g., rowaddresses 0 to 3k−1. In this manner, the three non-defective CAM blocks802(1)-802(3) of device 800 may be sold and operated as a 3k CAM array.This is in contrast to prior art CAM devices, which are typicallydiscarded if any of the CAM blocks therein are found to be defective.The ability to re-address the defective CAM block 802(0) and use thenon-defective CAM blocks 802(1)-802(3) of device 800 as a 3k CAM array,rather than discarding the device 800, may significantly increasemanufacturing yield.

In the example above, address space in the defective CAM block 802(0) istranslated from row assignments 0 to k−1 to row assignments 3k to 4k−1by changing the select value stored in the memory element 1006(0) from“0” to “3”. This ensures that the defective CAM block 802(0) will not beaddressed during read or write operations. That is, since the 3non-defective CAM blocks are used as a 3k CAM array having address space0 to 3k−1, address space higher than 3k−1 is not used, and therefore thedefective CAM block 802(0) will not be addressed. For an alternateembodiment, the block select signal BS_0 can be set to a low logic stateto disable block 802(0). For one example, the output of each MUX can becoupled to a logic circuit (e.g., one or more AND, OR, XOR, NOTcircuits) and memory 1006 configured to disable BS and its correspondingblock when a particular value is programmed into memory 1006 (or thevalue is changed in memory 1006).

In other embodiments, the block address values stored in memory elements1006(0)-1006(3) may be modified to translate address space in any numbern of non-defective CAM blocks into a contiguous address space of 0 to(k)n−1, irrespective of whether the non-defective CAM blocks areadjacent to one another. Thus, for example, if in one embodiment the CAMblocks 802(0) and 802(2) are defective and CAM blocks 802(1) and 802(3)are non-defective, the non-defective CAM blocks 802(1) and 802(3) may beconfigured for operation as a 2k CAM array by setting the block addressvalues for memory elements 1006(1) and 1006(3) to “0” and “1”,respectively. In this manner, the first 1k address space correspondingA[13:12] equal to “00” selects CAM block 802(1), and the second 1kaddress space corresponding to A[13:12] equal to “01” selects CAM block802(3). The block address values stored in memory elements 1006(0) and1006(2) each may be either “2” or “3” to preclude their selection duringoperation, since addresses above 2k−1, i.e., the third or fourth 1kaddress spaces corresponding to A[13:12] equal to “10” or “11”,respectively, are not used. Table 3 summarizes the block address valuesand corresponding address space when CAM blocks 802(0) and 802(2) aredefective and CAM blocks 802(1) and 802(3) are non-defective.

TABLE 3 Block Status MUX address space CAM 802(0) defective 2 or 3 >2k*CAM 802(1) non-defective 0 0 to k-1 CAM 802(2) defective 2 or 3 >2k* CAM802(3) non-defective 1 k to 2k-1 *not used

Each memory element 1006 may be any suitable structure to provide ablock address value to the corresponding multiplexer 1004 to select oneof lines 1008(0)-1008(3) to pass as the block select signal BS. In someembodiments, the memory element may be a flip-flop, register, look-uptable, or non-volatile memory such EPROM or Flash memory. In otherembodiments, the memory element 1006 may include one or more fuses toprovide the block address value to the corresponding multiplexer 1004.

For one example, in one embodiment of the CAM device 800, each memoryelement 1006 includes two fuses coupled to a voltage supply to initiallyprovide the binary value “11” to corresponding multiplexers 1004. Inthis example, since each multiplexer 1004 initially selects the signalline connected to its “3”input in response to the binary block addressvalue “11”, the “3” input of each multiplexer 1004 is connected to acorresponding numbered signal line 1008 from decode logic 1002. That is,input 3 of multiplexer 1004(0) is connected to the line 1008(0), inputnumber 3 of multiplexer 1004(1) is connected to the line 1008(1), inputnumber 3 of multiplexer 1004(2) is connected to the line 1008(2), andinput number 3 of multiplexer 1004(3) is connected to the line 1008(3).The remaining multiplexer inputs 0, 1, and 2 may be connected to lines1008 in any suitable configuration. In this manner, multiplexer 1004(0)asserts BS_0 when A[13:12] equals “00”, multiplexer 1004(1) asserts BS_1when A[13:12] equals “01”, multiplexer 1004(2) asserts BS_2 whenA[13:12] equals “10”, and multiplexer 1004(3) asserts BS_3 when A[13:12]equals “11”. Then, if after testing one or more of the CAM blocks 802are found to be defective, the fuses of each memory element 1008 may beselectively blown to translate address space from defective CAM blocksto non-defective CAM blocks 802 to facilitate contiguous addressing inthe manner described above. Additionally, the two logic ones may beANDed together and provided as one input to an AND gate, and the otherinput to the AND gate coupled to a respective BS signal output by eachMUX. When a defective row is programmed to a value other than “11”, thenthe respective BS signal and corresponding block will be disabled.

In order to maintain address consistency between read or writeoperations and compare operations when address space in a defective CAMblock is translated to a non-defective CAM block, address translationinformation used during the read or write operation is also used tocalculate the address or index of a matching CAM row during compareoperations. Thus, for example, if address space 0 to k−1 is translatedfrom CAM block 802(0) to 802(1), and there is a match in CAM block802(1) during a subsequent compare operation, the priority encoder 806ensures that the matching index from CAM block 802(1) lies withinaddress space 0 to k−1, rather than within address space k to 2k−1. Inthis manner, address translations facilitated during a read or writeoperation are reflected during subsequent compare operations.

Referring again to FIG. 9, during compare operations, a comparand wordprovided on CBUS is compared to data stored in all enabled (e.g.,non-defective) CAM blocks 802. For each enabled CAM block 802, if thereis a match condition in response to the compare operation, match flaglogic 805 asserts a match flag (MF_0 to MF_3) to a logic high state, andthe priority encoder 804 within the CAM block 802 outputs the 12-bit rowindex I (or address) of the highest priority matching CAM row in theblock. If there is not a match, the match flags are not asserted (i.e.,match flag logic 805 sets MF to low logic state). For one embodiment,each match flag logic includes a programmable element (e.g., a fuse orother memory element) that is programmed when the CAM block is disabled.The match flags MF_0 to MF_3 and row indexes I0-I3 from CAM blocks802(0)-802(3), respectively, are provided to the main priority encoder806. The main priority encoder 806 adds a unique block index to each rowindex I provided by CAM blocks 802(0)-802(3) to form a correspondingdevice index. The main priority encoder 806 uses the match flag signalsMF_0 to MF_3 to select the highest-priority device index from CAM blocks802(0)-802(3) to output as the system index, I_sys.

The main priority encoder 806 is programmable and stores the blockindexes for CAM blocks 802(0)-802(3) in memory (not shown in FIG. 9).The block indexes are dynamic values that may be modified or programmedto reflect and thus maintain consistency with address translations asdescribed above in a read or write operation. Initially, the mainpriority encoder stores a block index of “00” for CAM block 802(0), ablock index of “01” for CAM block 802(1), a block index of “10” for CAMblock 802(2), and a block index of “11” for CAM block 802(3). Theseinitial block indexes, which are used when all CAM blocks 802(0)-802(3)are enabled, mirror the block address values stored in memory elements1006(0)-1006(3) of address translation logic 1000 of FIG. 11. If one ormore CAM blocks 802 are found to be defective or are otherwise disabled,the block indexes stored in main priority encoder 806 are modified toreflect address translations during the read or write operation. Forexample, if CAM blocks 802(0) and 802(2) are disabled and address spacesin non-defective CAM blocks 802(1) and 802(3) are translated to addressspace 0 to k−1 and address space k to 2k−1, respectively, main priorityencoder 806 adds a block index of “00” to row index I1 to generate thedevice index for CAM block 802(1) and adds a block index “01” to rowindex I3 to generate the device index for CAM block 802(3). In thismanner, address consistency between read/write operations and compareoperations is maintained.

FIG. 12 shows a priority encoder 1200 that is one embodiment of the mainpriority encoder 806 of FIG. 9. The priority encoder 1200 includes achain of four multiplexers 1202(0)-1202(3), four corresponding memoryelements 1204(0)-1204(3), and select logic including an inverter 1206and OR gates 1208 and 1210. Each memory element 1204 stores a 2-bitblock index for a corresponding CAM block 802. The memory elements 1204may be the same as memory elements 1006 of FIG. 11, or they may beseparate memory elements. Each multiplexer 1202 includes a first input(i.e., the “1” input) coupled to the output of a preceding multiplexer1202 in the chain, a second input (i.e., the “0” input) to receive aconcatenation of a 12-bit index I of the highest priority match (if any)from a corresponding CAM block 802 and a 2-bit block index from thecorresponding memory element 1204, an output coupled to input “1” of anext multiplexer 1202, and a select terminal to receive matchinformation from the CAM blocks 802.

The concatenation of a 12-bit row index I and the 2-bit block indexforms a 14-bit device index of the highest-priority match, if any, froma corresponding CAM block 802. The 1 input of the first multiplexer1202(0) receives a default binary “0” value. MF_0 is inverted byinverter 1206 and provided as the select signal to multiplexer 1202(0),and provided directly as the select signal to multiplexer 1202(1). MF_0and MF_1 are combined in OR gate 1208 and provided as the select signalfor multiplexer 1202(2). MF_2 and the result from OR gate 1208 (i.e.,MF_0 +MF_1, where + is the logic OR function) are combined in OR gate1210 and provided as the select signal for multiplexer 1202(3). Asexplained below, the match flags MF control whether each multiplexer1202 passes a concatenated device index from a previous CAM block or theconcatenated device index of the corresponding CAM block.

In this example, CAM block 802(0) is the highest-priority block, CAMblock 802(1) is the next highest-priority block, and so on. For eachmultiplexer stage, if there is a match in the corresponding CAM block802, the row index I and block index are forwarded to the next stage ifthere is not a match condition in a previous or higher-priority CAMblock 802. If there is a match condition in a higher-priority CAM block802, the row index I plus block index from the higher-priority CAM blockare forwarded to the next stage.

For example, if there is a match condition in the first CAM block802(0), priority encoder 804 of CAM block 802(0) provides the 12-bit rowindex I0 of its highest-priority match to input 0 of multiplexer1202(0), where it is concatenated with the block index from memoryelement 1204(0) to generate the device index for CAM block 802(0). Thematch flag MF_0 is asserted to logic high to indicate the matchcondition. In response thereto, inverter 106 provides a logic low or “0”select signal to multiplexer 1202(0), which in turn forwards the deviceindex from CAM block 802(0) to the next multiplexer 1204(1). The logichigh MF_0 signal causes multiplexer 1204(1) to select input 1, and thusforwards the device index from CAM block 802(0) to the next multiplexer1202(2). MF_0 ripples through OR gates 1208 and 1210 and causesmultiplexers 1202(2) and 1202(3) to output the device index from CAMblock 802(0) as I_sys.

Maintaining equivalent values in corresponding memory elements1006(0)-1006(3) and 1204(0)-1204(3) ensures addressing consistencybetween read or write operations and compare operations. For instance,in one embodiment where all CAM blocks 802(0)-802(3) are non-defectiveor otherwise enabled, memory elements 1204(0)-1204(3) store values of“00”, “01”, “10” and “11”, respectively. In this manner, “00” is addedto row index I0 so that the first 1k addresses are mapped to the firstblock 802(0), “01” is added to row index I1 so that the second 1kaddresses are mapped to the second block 802(1), “10” is added to rowindex I2 so that the third 1k addresses are mapped to the third block802(2), and “11” is added to row index I3 from block 802(3) so that thefourth 1k addresses are mapped to the fourth block 802(3).

The block indexes stored in memory elements 1204(0)-1204(3) may bechanged when address space in one or more CAM blocks 802 is translatedto maintain addressing consistency. For example, in one embodiment whereCAM block 802(0) is defective, CAM blocks 802(1)-802(3) may beconfigured to operate as a 3k CAM array as described above with respectto address translation logic 1000 (FIG. 11) by setting block addressvalues or their binary equivalents of “0”, “1” and “2” into memoryelements 1006(1), 1006(2), and 1006(3), respectively. This configuresCAM block 802(1) to have the highest-priority address space, i.e.,addresses 0 to k−1, CAM block 802(2) to have the next highest-priorityaddress space, i.e., k to 2k−1, and CAM block 802(3) to have thelowest-priority address space, i.e., addresses 2k to 2k−1.

In accordance with present embodiments, the block indexes stored inmemory elements 1204(0)-1204(3) are modified to reflect addresstranslations facilitated in address translation logic 1000. Since CAMblock 802(0) is disabled (and thus does not require address space), theblock indexes of the remaining enabled CAM blocks 802(1)-802(3) may bemodified to re-assign block priority in the CAM device 800. For example,the block index stored in memory element 1204(1) may be set to “00” sothat when concatenated with row index I1 from the highest-priority CAMblock 802(1), the resultant device index corresponds to thehighest-priority address space, i.e., addresses 0 to k−1. Similarly, theblock indexes stored in memory elements 1204(2) and 1204(3) may bemodified to “01” and “10”, respectively, to reflect address spaces k to2k−1 and 2k to 3k−1, respectively. Since in this embodiment addresseslarger than 3k are not used, the block index stored in memory element1204(0), which corresponds to the defective CAM block 802(0), may be setto “11” so that disabled CAM block 802(0) is not addressed. Note thatthe match flag signal for a disabled CAM block will be set to a lowlogic state.

I claim:
 1. A content addressable memory (CAM) device comprising: aplurality of CAM blocks each having an array of CAM cells organized in anumber of rows and columns; and select means for selectively disablingone or more of the plurality of CAM blocks during a compare operation.2. The device of claim 1, wherein the CAM blocks comprise binary CAMs.3. The device claim 1, wherein the CAM blocks comprise ternary CAMs. 4.The device claim 1, further comprising a priority encoder coupled toeach of the plurality of CAM blocks.
 5. The device of claim 1, whereinthe select means comprises a plurality of block select circuits, each ofwhich corresponds to and selectively disables a corresponding CAM blockduring the compare operation.
 6. The device of claim 5, wherein eachblock select circuit comprises: a memory for storing a class code forthe corresponding CAM block; and a compare circuit for comparing asearch code with the class code to generate a select signal, the selectsignal for selectively disabling the corresponding CAM block during thecompare operation.
 7. The device of claim 6, wherein the memorycomprises a register.
 8. The device of claim 6, wherein the comparecircuit comprises one or more exclusive-OR type logic gates.
 9. Thedevice of claim 1, wherein each CAM block further comprises: a number ofcomparand drivers each coupled to a corresponding column of the arrayand coupled to receive comparand data.
 10. The device of claim 9,wherein each CAM block further comprises a number of complementarycomparand lines corresponding to the number of columns, the comparanddrivers coupled to the complementary comparand lines.
 11. A contentaddressable memory (CAM) device comprising: a plurality of CAM blockseach including an array of CAM cells; and a plurality of block selectcircuits, each comprising: a memory for storing a class code for acorresponding CAM block; and a compare circuit having a first input toreceive a search code, a second input to receive the class code from thememory, and an output coupled to the corresponding CAM block.
 12. Thedevice of claim 11, wherein each block select circuit for selectivelydisabling a corresponding CAM block during a compare operation inresponse to a comparison between the search code and the class code. 13.The device of claim 11, wherein each CAM block further comprises: anumber of comparand line drivers, each coupled to a corresponding columnof the array and to a corresponding block select circuit, each blockselect circuit for selectively disabling one or more comparand linedrivers of the corresponding CAM block.
 14. The device of claim 11,wherein each CAM block further comprises: an array of CAM cells arrangedin a plurality of rows and columns; a plurality of lines, each coupledto the CAM cells in a corresponding column of the array; a plurality ofinputs to receive a comparand word from a comparand bus; and a pluralityof comparand drivers, each coupled to a corresponding one of theplurality of lines and having an input terminal coupled to the blockselect circuit, wherein the plurality of comparand drivers forselectively driving a comparand word onto the lines in response to theselect signal provided by the block select circuit.
 15. The device ofclaim 14, wherein the lines comprise comparand lines.
 16. A method ofselectively disabling one or more of a plurality of content addressablememory (CAM) blocks of a CAM device from participating in a compareoperation between a comparand word and data stored in respective arraysof the CAM blocks, the method comprising: assigning a class code to eachCAM block; providing a search code to the CAM blocks; for each CAMblock, comparing the search code with the class code to generate aselect signal; and selectively disabling one or more of the CAM blocksin response to the select signal.
 17. The method of claim 16, whereinthe class codes define individually searchable partitions in the CAMdevice.
 18. The method of claim 16, wherein a unique class code isassigned to each CAM block.
 19. The method of claim 16, wherein a sameclass code is assigned to more than one CAM block.
 20. The method ofclaim 16, wherein the CAM block is disabled during the compare operationif the search code does not match the class code.
 21. The method ofclaim 16, wherein the CAM block is not disabled during the compareoperation if the search code matches the class code.
 22. The method ofclaim 16, wherein selectively disabling the CAM block comprisespreventing the comparand word from being driven into the array.
 23. Themethod of claim 22, wherein preventing the comparand word from beingdriven into its array comprises forcing a set of comparand lines to apredetermined state.
 24. A content addressable memory (CAM) devicecomprising: a plurality of CAM blocks each having an array of CAM cellsorganized in a number of rows and columns, and comprising a number ofcomplementary comparand lines corresponding to the number of columns;select means for selectively disabling one or more of the plurality ofCAM blocks during a compare operation; and a number of comparand driverseach coupled to a corresponding complementary comparand line and coupledto receive comparand data.
 25. The device of claim 24, wherein theselect means comprises a plurality of block select circuits, each ofwhich corresponds to and selectively disables a corresponding CAM blockduring the compare operation.
 26. The device of claim 25, wherein eachblock select circuit comprises: a memory for storing a class code forthe corresponding CAM block; and a compare circuit for comparing asearch code with the class code to generate a select signal, the selectsignal for selectively disabling the corresponding CAM block during thecompare operation.
 27. The device of claim 26, wherein the memorycomprises a register.
 28. A content addressable memory (CAM) devicecomprising: a plurality of CAM blocks each having an array of CAM cellsorganized in a number of rows and columns; a plurality of comparanddrivers each for providing comparand data to a corresponding CAM block;a plurality of memory circuits each for storing a class code indicativeof a class of data for storage in a corresponding CAM block; and aplurality of compare circuits each for comparing a search key with oneof the class codes for a corresponding CAM block, and operable to enablethe plurality of comparand drivers to provide the comparand data to thecorresponding CAM block in response to the comparison.
 29. The device ofclaim 28, wherein the compare circuits are further operable to disablethe plurality of comparand drivers from providing the comparand data tothe corresponding CAM block in response to the comparison.
 30. Thedevice of claim 29, wherein the disabled comparand drivers force thecomparand data to a predetermined state.
 31. A content addressablememory (CAM) device comprising: a first CAM block having an array of CAMcells organized in a number of rows and columns, and for storing data ofa first class type; a second CAM block having an array of CAM cellsorganized in a number of rows and columns, and for storing data of asecond class type; and means for enabling comparand data for comparisonwith the data of the first class type of the first CAM block and fordisabling the comparand data for comparison with the data of the secondclass type of the second CAM block.
 32. A method of operating a contentaddressable memory (CAM) device having a plurality of CAM blocks eachincluding an array of CAM cells, comprising: storing data of a firstclass type in a first CAM block; storing data of a second class type ina second CAM block; receiving comparand data associated with the firstclass type; and selectively comparing the comparand data only with dataof the first class type of the first CAM block.
 33. The method of claim32, wherein the selectively comparing comprises: enabling the comparanddata to be provided to the first CAM block; and disabling the comparanddata from being provided to the second CAM block.